Network access of a wireless device to a communications network

ABSTRACT

A communications network is being accessed by a wireless device associated with a coverage class selected from a set of coverage classes. The wireless device performs a method comprising initiating network access to the communications network by transmitting a preamble sequence for random access on a physical random access channel during a starting opportunity defined by the coverage class of the wireless device. Each coverage class may be associated with a unique number of repetitions of the preamble sequence transmission.

PRIORITY

This non-provisional application claims priority to U.S. ProvisionalPatent Application No. 62/309,389 filed Mar. 16, 2016, which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments presented herein relate to a method, a wireless device, acomputer program, and a computer program product for network access ofthe wireless device to a communications network. Further embodimentspresented herein relate to a method, a network node, a computer program,and a computer program product for enabling network access of thewireless device to the communications network.

BACKGROUND

In communications networks, there may be a challenge to obtain goodperformance and capacity for a given communications protocol, itsparameters and the physical environment in which the communicationsnetwork is deployed.

For example, evolving services are associated with new requirements oncellular networks, e.g. with respect to device cost, battery lifetimeand coverage. To drive down device and module cost, a system-on-a-chip(SoC) solution with integrated power amplifier (PA) can be used.However, it is feasible for the current state-of-the-art PA technologyto allow 20-23 dBm transmit power when PA is integrated to SoC. Thisconstraint limits uplink coverage, which is related to how much the pathloss is allowed between the end-user wireless device and network node ofthe communications network.

To maximize the coverage achievable by an integrated PA, it is commonlynecessary to reduce PA backoff. PA backoff may be defined as the ratioof maximal saturation output power and average output power of the PA.PA backoff is needed when the communication signal has significantnon-unity peak-to-average power ratio (PAPR). The higher the PAPR is,the higher PA backoff is required. Higher PA backoff also gives rise tolower PA efficiency, and thus lower device battery life time. Thus,designing an uplink communication signal that has as low PAPR aspossible—and thereby reduces the necessary PA backoff—could lower thedevice cost, increase the battery lifetime and increase the coverage ofthe wireless device.

It could be possible to evolve existing cellular communications (such asLong Term Evolution; LTE) specifications to include support forNarrowband Internet-of-Things (NB-IoT) technologies. In this respect,the LTE uplink is based on single-carrier frequency-divisionmultiple-access (SC-FDMA) modulation for the uplink data and controlchannels, and Zadoff-Chu signal for random access. Neither of thesesignals has good PAPR properties.

Hence, there is still a need for an improved handling of network accessfor a wireless device in a communications network.

SUMMARY

An object of embodiments herein is to provide efficient handling ofnetwork access for a wireless device in a communications network

According to a first aspect there is presented a method for networkaccess of a wireless device to a communications network. The wirelessdevice is associated with a coverage class from a set of coverageclasses. The method is performed by the wireless device. The methodcomprises initiating network access to the communications network bytransmitting a preamble sequence for random access on a physical randomaccess channel (PRACH). Wherein the network access is initiated during astarting opportunity defined by the coverage class of the wirelessdevice.

According to a second aspect there is presented a wireless device fornetwork access of the wireless device to a communications network. Thewireless device is associated with a coverage class from a set ofcoverage classes. The wireless device comprises processing circuitry.The processing circuitry is configured to cause the wireless device toinitiate network access to the communications network by transmitting apreamble sequence for random access on a physical random access channel(PRACH) The network access is initiated during a starting opportunitydefined by the coverage class of the wireless device.

According to a third aspect there is presented a wireless device fornetwork access of the wireless device to a communications network. Thewireless device is associated with a coverage class from a set ofcoverage classes. The wireless device comprises processing circuitry anda computer program product storing instructions that, when executed bythe processing circuitry, causes the wireless device to initiate networkaccess to the communications network by transmitting a preamble sequencefor random access on a physical random access channel (PRACH). Thenetwork access is initiated during a starting opportunity defined by thecoverage class of the wireless device.

According to a fourth aspect there is presented a wireless device fornetwork access of the wireless device to a communications network. Thewireless device is associated with a coverage class from a set ofcoverage classes. The wireless device comprises an initiate moduleconfigured to initiate network access to the communications network bytransmitting a preamble sequence for random access on a physical randomaccess channel (PRACH). The network access is initiated during astarting opportunity defined by the coverage class of the wirelessdevice.

According to a fifth aspect there is presented a computer program fornetwork access of a wireless device to a communications network, thecomputer program comprising computer program code which, when run onprocessing circuitry of the wireless device, causes the wireless deviceto perform a method according to the first aspect.

According to a sixth aspect there is presented a method for enablingnetwork access of a wireless device to a communications network. Thewireless device is associated with a coverage class from a set ofcoverage classes. The method is performed by a network node. The methodcomprises providing a network access configuration to the wirelessdevice. The network access configuration specifies network accessinitiation to the communications network for the wireless device. Thenetwork access configuration specifies a starting opportunity defined bythe coverage class of the wireless device during which network access isto be initiated.

According to a seventh aspect there is presented a network node forenabling network access of a wireless device to a communicationsnetwork. The wireless device is associated with a coverage class from aset of coverage classes. The network node comprises processingcircuitry. The processing circuitry is configured to cause the networknode to provide a network access configuration to the wireless device.The network access configuration specifies network access initiation tothe communications network for the wireless device. The network accessconfiguration specifies a starting opportunity defined by the coverageclass of the wireless device during which network access is to beinitiated.

According to an eighth aspect there is presented a network node forenabling network access of a wireless device to a communicationsnetwork. The wireless device is associated with a coverage class from aset of coverage classes. The network node comprises processing circuitryand a computer program product storing instructions that, when executedby the processing circuitry, causes the network node to provide anetwork access configuration to the wireless device. The network accessconfiguration specifies network access initiation to the communicationsnetwork for the wireless device. The network access configurationspecifies a starting opportunity defined by the coverage class of thewireless device during which network access is to be initiated.

According to a ninth aspect there is presented a network node forenabling network access of a wireless device to a communicationsnetwork. The network node comprises a provide module configured toprovide a network access configuration to the wireless device. Thenetwork access configuration specifies network access initiation to thecommunications network for the wireless device. The network accessconfiguration specifies that the network access is to be initiatedduring a starting opportunity defined by the coverage class of thewireless device.

According to a tenth aspect there is presented a computer program forenabling network access of a wireless device to a communicationsnetwork, the computer program comprising computer program code which,when run on processing circuitry of a network node, causes the networknode to perform a method according to the sixth aspect.

According to an eleventh aspect there is presented a computer programproduct comprising a computer program according to at least one of thefifth aspect and the tenth aspect and a computer readable storage mediumon which the computer program is stored. The computer readable storagemedium can be a non-transitory computer readable storage medium.

Advantageously these methods and devices provide efficient handling ofnetwork access of the wireless device to the communications network.

Advantageously these methods and devices enable time collision of PRACHopportunities of different coverage classes to be avoided.

Advantageously these methods and devices particularly apply to NB-IoTand Enhanced Machine-Type Communication (eMTC).

It is to be noted that any feature of the first, second, third, fourth,fifth, sixth seventh, eighth, ninth, tenth and eleventh aspects may beapplied to any other aspect, wherever appropriate. Likewise, anyadvantage of the first aspect may equally apply to the second, third,fourth, fifth, sixth, seventh, eighth, ninth, tenth, and/or eleventhaspect, respectively, and vice versa. Other objectives, features andadvantages of the enclosed embodiments will be apparent from thefollowing detailed disclosure, from the attached dependent claims aswell as from the drawings.

Generally, all terms used in the claims are to be interpreted accordingto their ordinary meaning in the technical field, unless explicitlydefined otherwise herein. All references to “a/an/the element,apparatus, component, means, step, etc.” are to be interpreted openly asreferring to at least one instance of the element, apparatus, component,means, step, etc., unless explicitly stated otherwise. The steps of anymethod disclosed herein do not have to be performed in the exact orderdisclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concept is now described, by way of example, withreference to the accompanying drawings, on which:

FIG. 1 is a schematic diagram illustrating a communication networkaccording to embodiments;

FIG. 2 schematically illustrates reception of random access preambles ata radio access network node;

FIG. 3 schematically illustrates a PRACH symbol group structure;

FIG. 4 schematically illustrates a PRACH hopping pattern;

FIG. 5 schematically illustrates a 12-tone (12-subcarrier) NPRACH band;

FIG. 6-7 schematically illustrates PRACH opportunities according toprior art;

FIGS. 8-12 schematically illustrate PRACH opportunities according toembodiments;

FIGS. 13-16 are flowcharts of methods according to embodiments;

FIG. 17 is a schematic diagram showing functional units of a wirelessdevice according to an embodiment;

FIG. 18 is a schematic diagram showing functional modules of a wirelessdevice according to an embodiment;

FIG. 19 is a schematic diagram showing functional units of a networknode according to an embodiment;

FIG. 20 is a schematic diagram showing functional modules of a networknode according to an embodiment; and

FIG. 21 shows one example of a computer program product comprisingcomputer readable means according to an embodiment.

Unless otherwise stated, like references numerals indicate like elementson the drawings.

DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter withreference to the accompanying drawings, in which certain embodiments ofthe inventive concept are shown. This inventive concept may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided by way of example so that this disclosure will be thorough andcomplete, and will fully convey the scope of the inventive concept tothose skilled in the art. Like numbers refer to like elements throughoutthe description. Any step or feature illustrated by dashed lines shouldbe regarded as optional.

Single-tone frequency-hopping NB-IoT PRACH (denoted NPRACH) signals havelow PAPR, and thus the use of NPRACH reduces the need for PA backoff andmaximizes PA efficiency. NPRACH signals are compatible with SC-FDMA andorthogonal frequency-division multiple-access (OFDMA) since, in any OFDMsymbol interval, the NPRACH signals appear like an OFDM signal of onesingle subcarrier.

To support the random access design, the network node should be able toconfigure time resource information that informs the wireless deviceswhen (in time) to transmit the NPRACH and frequency resource informationthat directs wireless devices where (in frequency) to transmit theNPRACH. In NB-IoT random access, up to three different coverage classesmay be supported in NPRACH. Coverage classes are also referred to ascoverage levels, coverage enhancement levels (CE levels or CELs), orenhanced coverage levels; the term coverage classes will be usedhereinafter. For example, coverage classes may correspond to a value ofa minimum coupling loss (MCL), which may signify a minimum distanceloss—possibly including antenna gain—measured between antennaconnectors, such as 144 dB MCL or 164 dB MCL. More generally, coverageclasses may correspond to x dB MCL, where x is selected from apredetermined collection of two or more values, e.g. {144, 164}.Coverage classes may alternatively be associated with respective valuesof a received power of a signal which the wireless device receives, inparticular a reference signal. As discussed in more detail below,coverage classes may correspond to the number of repetitions of anNPRACH signal that a UE transmits.

The received powers of NPRACH transmissions from wireless devices indifferent coverage classes can differ significantly, resulting in asevere near-far problem if the transmissions use the same time andfrequency NPRACH opportunities. As an example, supporting 164 dB maximumcoupling loss is a design target of NB-IoT, while the maximum couplingloss of wireless devices in normal coverage is often limited to 144 dB.This may result in 20 dB received power difference under the idealconditions that the wireless devices are able to estimate their couplingloss perfectly and the open loop power control used in NPRACHtransmissions is perfect. In practice, the estimate of coupling loss bythe wireless devices may be subject to errors in, e.g., the range [−6,6] dB, leading to even larger received power differences in NPRACHtransmissions. It is therefore proposed to separate NPRACH opportunitiesof different coverage classes in time and/or frequency domain.

One alternative to separating NPRACH opportunities of different coverageclasses is to configure different NPRACH frequency bands for differentcoverage classes in the frequency domain. However, if the network nodeis to configure only one or two NPRACH frequency bands, a mechanism isstill needed to separate the NPRACH of three different coverage classesin the time domain.

In the existing LTE random access procedure, random access servesmultiple purposes such as initial network access when a radio link isestablished between the wireless device and the communications network,scheduling request for the wireless device, etc. Among others, oneobjective of random access is to achieve uplink synchronization formaintaining the uplink orthogonality in LTE. To preserve orthogonalityamong different wireless devices in an OFDMA or SC-FDMA system, the timeof arrival of each wireless device's signal needs to be within thecyclic prefix (CP) of the OFDMA or SC-FDMA signal at the network node.

LTE random access can be either contention-based or contention-free. Thecontention-based random access procedure consists of four steps, asillustrated in FIG. 1.

FIG. 1 is a schematic diagram illustrating a communications network 100where embodiments presented herein can be applied. The communicationsnetwork 100 comprises a radio access network 110, a core network 120 anda service network 130. The radio access network 100 comprises at leastone radio access network node (RANN) 140. The radio access network node140 can be provided by any of a radio base station, a base transceiverstation, a remote radio head, an access point, an access node, a NodeB,or an evolved NodeB. The radio access network node 140 provides servicesand network access to at least one wireless device (WD) 200. Thewireless device 200 can be a portable wireless device, a mobile station,a mobile phone, a handset, a wireless local-loop phone, a user equipment(UE), a smartphone, a laptop computer, a tablet computer, a networkequipped sensor device, an Internet-of-Things device, or a wirelessbroadband modem.

The radio access network 110 is operatively connected to the corenetwork 120, which in turn is operatively connected to the servicenetwork 130. A wireless device 200 being operatively connected to theradio access network node 140 is thereby enabled to access services andexchange data with the service network 130.

The communications network 100 further comprises at least one networknode 300. Further details of the network node 300 will be disclosedbelow.

The contention-based random access procedure comprises steps 1-4:

Step 1: The wireless device 200 transmits a random access preamble tothe network node 300.

Step 2: The network node 300 responds to the random access preamble bytransmitting a random access response including, for example, an uplinkgrant, to the wireless device 200.

Step 3: The wireless device 200 transmits a scheduled transmission tothe network node 300.

Step 4: The network node 300 transmits a message for contentionresolution of the wireless device 200.

Note that only step 1 involves physical-layer processing specificallydesigned for random access, while the remaining steps 2-4 follow thesame physical-layer processing used in uplink and downlink datatransmission. For contention-free random access, the wireless deviceuses reserved preambles assigned by the base station. In this case,contention resolution is not needed, and thus only steps 1 and 2 arerequired.

NPRACH serves similar purposes as in LTE, and reuses the random accessprocedure in LTE. As shown in FIG. 1, in the first step, a PRACHpreamble sequence is sent by the UE during a random access time segmentillustrated in FIG. 2. For a wireless device close to the radio accessnetwork node the random access preamble is received at time t=t₁. For awireless device at the cell edge (far from the radio access networknode) the random access preamble is received at time t=t₁+Δ. The PRACHpreamble sequence does not occupy the entire random access segment,leaving some time as guard time (GT). As discussed above, to maximize PAefficiency and coverage, it is desirable to have PRACH preambles asclose to constant-envelope as possible. Also, the PRACH preambles shouldbe designed such that accurate time-of-arrival estimation can beperformed by the base stations. In the below description, the termsPRACH signal and PRACH preamble will be used interchangeably.

One example of the basic structure of a PRACH symbol group isillustrated in FIG. 3. It is basically a single tone OFDM signal. Unliketraditional OFDM symbols where the non-CP part consists of a singlesymbol, the non-CP part of the PRACH symbol group in FIG. 3 may consistof one or more symbols. As an example, one CP (of length either 266.7 μsor 66.7 μs) and five symbols constitute a basic symbol group. The symbolstructure with 266.7 μs CP and five symbols is illustrated in FIG. 3.

A number of OFDM symbol groups, each one as illustrated in FIG. 3, areconcatenated to form a PRACH preamble. But the frequency positions ofthe symbol groups of the same PRACH preamble vary according to somehopping patterns. One example of a hopping pattern is illustrated inFIG. 4.

Based on using single-tone frequency-hopping NPRACH, 12 tones (of totalbandwidth 3.75·12=45 kHz) can be used as the basic frequency resourceband (such as 6 PRBs in LTE PRACH) for the configuration design. This12-tone NPRACH band concept is illustrated in FIG. 5.

For wireless devices in normal coverage, a NPRACH preamble transmissionwith 4 or 8 symbol groups can be sufficient in order for the wirelessdevice to successfully complete the random access procedure. Forwireless devices in extreme low coverage with e.g., 164 dB maximumcoupling loss, a NPRACH preamble transmission with 128 or more symbolgroups may be required.

It could be beneficial to avoid collision of NPRACH transmissions fromwireless devices in different coverage classes when they use the sameNPRACH frequency band. Mechanisms are therefore proposed to separateNPRACH opportunities of different coverage classes in the time domain.

Assume, for example as in eMTC, that the range for the RRC parameter forPRACH starting subframe periodicity (expressed in terms of PRACHopportunities) is defined by prachStartingSubframe, which may take oneof the values in the set {2, 4, 8, 16, 32, 64, 128, 256}, whereprachStartingSubframe is a parameter defining the PRACH startingsubframe. Further assume that the offset (expressed in terms of PRACHopportunities) is defined byN·prachStartingSubframe+numRepetitionPerPreambleAttempt,where N≧0 is an integer, and where numRepetitionPerPreambleAttempt is aparameter defining the number of repetitions of random accesstransmissions allowed per preamble attempt. In 3GPP specifications ofeMTC, the initial transmission of a preamble sequence is counted in the“number of repetitions”; for example, transmitting a preamble sequencetwice per attempt may correspond to numRepetititionsPerPreambleAttemptbeing equal to 2. The total number of repetitions may depend on thenumber of allowed preamble transmission attempts, which may correspondto a different parameter.

Here prachStartingSubframe is expressed in terms of PRACH opportunities,not the absolute time or number of symbol groups. Time-domain PRACHopportunities can be regarded as slots in the time domain resources thatcan be used for PRACH transmissions, as illustrated in FIG. 6.Time-domain resources between slots may be used for other purposes, suchas data transmissions. Since time-domain resources between PRACHopportunities are not relevant for the embodiments disclosed herein,such time-domain resources will be omitted in below referenced FIGS.7-12.

A non-limiting illustrative example will be used to illustrate how therandom access procedure for eMTC can be used for NB-IoT. As an example,suppose that there are 16 opportunities every 128 ms. Consider threedifferent coverage classes, denoted coverage class 1, coverage class 2,and coverage class 3 with properties listed below:

For coverage class 1, 4 symbol groups are used (i.e., no repetition withrespect to a set of 4 symbol groups; this may correspond to a value 1 ofa repetition parameter) and can be transmitted completely using onePRACH opportunity.

For coverage class 2, 8 symbol groups are needed (i.e., 2 repetitionswith respect to a set of 4 symbol groups; this may correspond to a value2 of a repetition parameter) and can be transmitted using 2 PRACHopportunities.

For coverage class 3, 32 symbol groups are needed (i.e., 8 repetitionswith respect to a set of 4 symbol groups; this may correspond to a value8 of a repetition parameter) and can be transmitted using 8 PRACHopportunities.

In principle, in eMTC different prachStartingSubframe are allowed fordifferent coverage classes. This however may complicate the networkconfiguration for avoiding PRACH collisions of wireless devices indifferent coverage classes. One example of such a case is provided belowand is also illustrated in FIG. 7.

Coverage class 2 has 2 repetitions: prachStartingSubframe=4 and thus theoffset is any value in the set {0, 1, 2, 3}·4+2={2, 6, 10, 14} within 16opportunities. As seen in FIG. 7, N=0, 1, 2 or 3 for coverage class 2.FIG. 7 shows four preamble transmission attempts with two repetitionseach.

Coverage class 3 has 8 repetitions: prachStartingSubframe=16 and thusthe offset is 0·16+8=8 within 16 opportunities. As seen in FIG. 7, N=0for coverage class 3.

The embodiments disclosed herein relate to mechanisms for handling timecollision of PRACH opportunities of different coverage classes.

The embodiments disclosed herein thus relate to mechanisms for networkaccess of a wireless device to a communications network. In order toobtain such mechanisms there is provided a wireless device 200, a methodperformed by the wireless device 200, a computer program productcomprising code, for example in the form of a computer program, thatwhen run on processing circuitry of the wireless device 200, causes thewireless device 200 to perform the method. In order to obtain suchmechanisms there is further provided a network node 300, a methodperformed by the network node 300, and a computer program productcomprising code, for example in the form of a computer program, thatwhen run on processing circuitry of the network node 300, causes thenetwork node 300 to perform the method.

FIGS. 13 and 14 are flow charts illustrating embodiments of methods fornetwork access of the wireless device 200 to the communications network100 as performed by the wireless device 200. FIGS. 15 and 16 are flowcharts illustrating embodiments of methods for enabling network accessof the wireless device 200 to the communications network 100 asperformed by the network node 300. The methods are advantageouslyprovided as computer programs 420 a, 420 b.

Reference is now made to FIG. 13 illustrating a method for networkaccess of the wireless device 200 to the communications network 100 asperformed by the wireless device 200 according to an embodiment.

The wireless device 200 is associated with a coverage class from a setof coverage classes. Preferably, the set comprises two, three or morecoverage classes.

S110: The wireless device 200 initiates network access to thecommunications network 100 by transmitting a preamble sequence forrandom access on a physical random access channel (PRACH). The networkaccess is initiated during a starting opportunity defined by thecoverage class of the wireless device 200. Accordingly, the preamblesequence is transmitted during the starting opportunity and possiblyrepeated.

Reference is now made to FIG. 14 illustrating methods for network accessof the wireless device 200 to the communications network 100 asperformed by the wireless device 200 according to further embodiments.It is assumed that step S110 is performed as described with reference toFIG. 13.

There could be different ways for the wireless device 200 to obtain thenetwork access configuration. For example, according to an embodimentthe wireless device 200 is configured to perform step S102.

S102: The wireless device 200 obtains the network access configurationfrom the network node 300.

As will be further disclosed below the wireless device 200 can beprovided with information from the network node 300 on how many of thecoverage classes in the set of coverage classes share the frequency bandof the coverage class of the wireless device 200. According to anembodiment the wireless device 200 is therefore configured to performstep S104.

S104: The wireless device 200 obtains information from the network node300 on how many of the coverage classes in the set of coverage classesshare the frequency band of the coverage class of the wireless device200.

Reference is now made to FIG. 15 illustrating a method for enablingnetwork access of the wireless device 200 to the communications network100 as performed by the network node 300 according to an embodiment.

As disclosed above, the wireless device 200 is associated with acoverage class from a set of coverage classes.

S202: The network node 300 provides a network access configuration tothe wireless device 200. The network access configuration specifiesnetwork access initiation to the communications network 100 for thewireless device 200. The network access configuration specifies that thenetwork access is to be initiated during a starting opportunity definedby the coverage class of the wireless device 200. According to anembodiment, the network node 300 provides the wireless device 200 with anetwork access configuration including a plurality of startingopportunities associated with different coverage classes. The startingopportunities may relate to distinct time resources. Among the startingopportunities, the coverage class of the wireless device 200 defines astarting opportunity during which the wireless device 200 is to initiatenetwork access by transmitting a preamble sequence for random access ona physical random access channel. This may imply that the wirelessdevice 200 receives a network access configuration which, in addition tothe one or more starting opportunities of the wireless device 200,specifies at least one further starting opportunity; the furtherstarting opportunity may be used by wireless devices associated with adifferent coverage class.

Reference is now made to FIG. 16 illustrating methods for enablingnetwork access of the wireless device 200 to the communications network100 as performed by the network node 300 according to furtherembodiments. It is assumed that step S202 is performed as described withreference to FIG. 15.

According to an embodiment the network node 300 is configured to performstep S204:

S204: The network node 300 provides information to the wireless device200 on how many of the coverage classes in the set of coverage classesshare the frequency band of the coverage class of the wireless device200.

Embodiments common to both the wireless device 200 and the network node300 will now be presented.

With reference again made to FIG. 1, the network access initiation asperformed by the wireless device 200 in step S110 (FIGS. 13 and 14)takes the place of transmitting the random access preamble in step 1. Itcan be assumed that the preamble sequence for random access astransmitted by the wireless device 200 is received by the network node300. Steps 2-4 of FIG. 1 may then follow; in case contention resolutionis not needed only steps 1 and 2 are required to be performed.

According to some aspects the starting opportunities are unique for eachcoverage class. Hence, according to an embodiment no two differentcoverage classes in the set of coverage classes share a common startingopportunity.

According to some other aspects the starting opportunities are sharedfor some coverage classes. Particularly, according to an embodiment eachcoverage class in the set of coverage classes is associated with arespective received power level, and those coverage classes whosereceived power levels differ less than a threshold value have at leastpartly overlapping starting opportunities.

According to an embodiment all coverage classes in the set of coverageclasses share a common starting subframe for initiating the networkaccess, and the starting opportunity, during which the network access isinitiated, is determined based on the common starting subframe. For easeof avoiding collision, a common prachStartingSubframe can thus beconfigured. Then, collisions can be avoided since different offsets canbe used for different coverage classes. As an example, different timeoffsets relative to the common starting subframe can be used. In otherwords, the offset can be dependent on the coverage class, and eachcoverage class can have its own offset. That is, according to anembodiment all coverage classes in the set of coverage classes share acommon starting subframe for initiating said network access, and eachcoverage class in the set of coverage classes has a unique offset forinitiating the network access in relation to the common startingsubframe, and the network access is initiated according to the uniqueoffset. The offset may be defined in relation to the common startingsubframe. The offset may for instance include a number of subframesindicating the distance in time from the common starting subframe to thestarting opportunity. The offset may alternatively include a distance intime from the common starting subframe to the starting opportunity.Thus, if the offset for a coverage class is zero, one startingopportunity will be the common starting subframe, so that wirelessdevices associated with the coverage class will try to initiate networkaccess during the common starting subframe.

Embodiments relating to determination of explicit starting opportunitiesused by the wireless device 200 for avoiding PRACH collision will now bedisclosed.

According to an embodiment each coverage class in the set of coverageclasses is associated with a unique number of repetitions for performingthe network access. Like for eMTC, as reviewed above, the number ofrepetitions may be signaled by a parameter indicating the number ofrepetitions per attempt and an optional parameter indicating the numberof attempts. While network access may be initiated in a subframecontaining the starting opportunity, the subsequent repetitions may beoutside this subframe. The unique offset for a given coverage class inthe set of coverage classes is proportional to the unique number ofrepetitions for the given coverage class. Continuing the runningnon-limiting example, if the network node specifies the startingsubframe as prachStartingSubframe=16, in the first period with N=0,then:

Coverage class 1 has no repetitions (repetition parameter=1) and thusoffset=0·16+1=1,

Coverage class 2 has 2 repetitions (repetition parameter=2) and thusoffset=0·16+2=2, and

Coverage class 3 has 8 repetitions (repetition parameter=8) and thusoffset=0·16+8=8.

The PRACH opportunities for the three coverage classes are illustratedin FIG. 8. FIG. 8 illustrates use of three different offsets. As alsoshown in FIG. 8, network access may be initiated in one subframe(starting opportunity) but may continue (e.g., by one or more repeatedtransmissions of the preamble sequence) into subsequent subframes. Onedrawback is that not all the PRACH opportunities can be used.

According to an embodiment, each of the coverage classes in the set ofcoverage classes is associated with a unique number of startingopportunities. Further, each coverage class in the set of coverageclasses can be associated with a unique number of repetitions forinitiating the network access. Network access may be initiated in saidsubframe but subsequent subframes may contain one or more transmissionsrepeating a same random access preamble sequence. The number of startingopportunities for a coverage class with relatively fewer repetitions isthen higher than the number of starting opportunities for a coverageclass with relatively many repetitions. To fully utilize the PRACHopportunities, the following scheduling of the starting opportunitiescould thus be used:

Coverage class 1 has starting subframes indexed j·P+k·N_(rep,1), fork=0, 1, . . . , (P/(4·N_(rep,1))−1). Here N_(rep,1) is the number ofrepetitions of coverage class 1, and j is a sequence number of theperiod (time interval of length P). Transmitting the preamble sequenceonce may correspond to N_(rep,1)=1.Coverage class 2 has starting subframes (j+¼)·P+k·N_(rep,2), for k=0, 1,. . . , (P/(4·N_(rep,2))−1). Here N_(rep,2) is the number of repetitionsof coverage class 2.

Coverage class 3 has starting subframes (j+½)·P+k·N_(rep,3), for k=0, 1,. . . , (P/(2·N_(rep,3))−1). Here N_(rep,3) is the number of repetitionsof coverage class 3.

In the above, it can be assumed that the PRACH of all three coverageclasses share the same PRACH frequency band. Using the above schedulingof the starting opportunities, the number of PRACH opportunities in aperiod of length P is a function of the number of the repetitions of thegiven coverage class.

For the running non-limiting example, the following startingopportunities are obtained:

Coverage class 1 has no repetition and starting opportunities insubframes 0, 1, 2, 3,

Coverage class 2 has 2 repetitions and starting opportunities insubframes 4, 6, and

Coverage class 3 has 8 repetitions and starting opportunities insubframe 8.

The PRACH opportunities for the three coverage classes are illustratedin FIG. 9. One drawback is that all the PRACH opportunities for a givencoverage class are clustered.

It can be assumed that the network access is initiated in a frequencyband. According to an embodiment the starting opportunity is dependenton how many of the coverage classes in the set of coverage classes sharethe frequency band of the coverage class of the wireless device. Thewireless device 200 can then be provided with information from thenetwork node 300 on how many of the coverage classes in the set ofcoverage classes share the frequency band of the coverage class of thewireless device 200, as in above steps S104, S204. For example, if onlytwo coverage classes share the same PRACH frequency band, and P is thecommon prachStartingSubframe, then only those coverage classes sharingthe same PRACH frequency band need to be considered. For example, ifcoverage class 1 and coverage class 3 share the same PRACH frequencyband A, but coverage class 2 uses a separate PRACH frequency band B,then the following scheduling of the starting opportunities could beused:

Coverage class 1 has starting subframes j·P_(bandA)+k·N_(rep,1), fork=0, 1, . . . , (P_(bandA)/(2·N_(rep,1))−1). Here N_(rep,1) is thenumber of repetitions of coverage class 1, and j is again a sequencenumber of the period (time interval of length P).

Coverage class 2 has starting subframes j·P_(bandB). Parameter P_(bandB)can be set with value P_(bandB) N_(rep,2), with P_(bandB)=N_(rep,2)allowing the maximum number of PRACH opportunities for coverage class 2.

Coverage class 3 has starting subframes (j+½)·P_(bandA)+k·N_(rep,3), fork=0, 1, . . . , (P_(bandA)/(2·N_(rep,3))−1). Here N_(rep,3) is thenumber of repetitions of coverage class 3.

Since coverage class 2 uses a single PRACH frequency band, there is nocollision. For coverage class 1 and coverage class 3, collision can beavoided as illustrated in FIG. 10.

According to an embodiment, there are several possible startingopportunities. These several possible starting opportunities could thenbe distributed in time such that at least two of the several possiblestarting opportunities are separated by a starting opportunity ofanother coverage class in the set of coverage classes. Hence, thestarting opportunities could be scheduled so as to distribute the PRACHopportunities over time. This is illustrated in FIG. ii. The wirelessdevice 200 becomes aware of this configuration by receiving informationfrom the network node 300 that one coverage class defines severalpossible starting opportunities between which at least one startingopportunity defined by a different coverage class is interposed.

Embodiments relating to determination of implicit starting opportunitiesused by the wireless device 200 for avoiding PRACH collision based onranking of the wireless device 200 according to its coverage class andassigning a starting opportunity priority accordingly will now bedisclosed. A ranking of wireless devices may arise from the fact that,on the one hand, each wireless device 200 is associated with a coverageclass and, on the other hand, the coverage classes in the set ofcoverage classes are ranked in the sense that a given first coverageclass is either higher, lower or identical to a given second coverageclass.

According to an embodiment the starting opportunity is defined by thewireless device 200 backing off from initiating the network access. (Forthe avoidance of any doubt, it is pointed out that “backing off” in thissense is generally unrelated to the concept of “PA backoff” discussedinitially.) The wireless device 200 may perform said backing-off on thebasis of possible starting opportunities. If the wireless device 200 hasbacked off from initiating the network access in one possible startingopportunity, it will then determine whether to initiate the networkaccess in a next possible starting opportunity. As a result, no twodifferent coverage classes will share a common starting opportunity.

In more detail, according to aspects disclosed herein, the scheduling ofthe starting opportunities is based on a ranking approach. Specifically,the wireless devices are ranked according to their coverage class. Then,a higher coverage class is given higher priority over a lower coverageclass. Hence, according to an embodiment, each coverage class in the setof coverage classes has its own set of starting opportunities fornetwork access. In other words, when wireless devices in a lowercoverage class determine their starting opportunities, they back offfrom those starting opportunities that may be used by wireless devicesin higher coverage classes. Hence, according to an embodiment, allcoverage classes in the set of coverage classes are ranked, and thewireless device backs off from a starting opportunity used by wirelessdevices in a higher ranked coverage class. The wireless device isthereby enabled to determine the next available subframe (containing thePRACH) permitted by the restrictions given by the subframes containingPRACH resources of a higher enhanced coverage class.

Continuing the non-limiting illustrative example, the following startingopportunities are scheduled as followed (as illustrated in FIG. 12):

Coverage class 3 has 8 repetitions: prachStartingSubframe=16 and thusoffset=0·16+8=8 within 16 starting opportunities. Coverage class 3 hasthe highest priority and does not need to back off from any NPRACHresource.

Coverage class 2 has 2 repetitions: prachStartingSubframe=4 and thusoffset={0, 1, 2, 3}·4+2={2, 6, 10, 14} within 16 starting opportunities.Coverage class 2 has lower priority than coverage class 3. Wirelessdevices in this coverage class then check the possible startingopportunities used by coverage class 3, and find that offset {2, 3}would lead to collision and thus back off from the collided PRACHresource.

Hence, with reference to FIG. 14, according to an embodiment, thewireless device is configured to perform steps S106 and S108, asfollows.

S106: The wireless device checks possible starting opportunities of itsown coverage class and of a higher ranked coverage class.

S108: The wireless device backs off from initiating network access inany of its possible starting opportunities that may be used by thehigher coverage class.

Coverage class 1 has no repetitions (which may be represented by a value1 of a repetition parameter) and potentially can use any PRACHopportunity with the following restriction. Coverage class 1 has thelowest priority. Wireless devices in this coverage class then check thepossible starting opportunities used by coverage class 2 and coverageclass 3, and back off from the PRACH resources that collide withcoverage class 2 and coverage class 3 as a result of performing stepsS106 and S108.

The ranking approach allows different prachStartingSubframe to beconfigured for different coverage classes. The ranking approach allowsfull utilization of all the PRACH opportunities. The ranking approachallows clustering of PRACH opportunities of any coverage class to beavoided. Furthermore, since the task of finding the available startingopportunities among the possible starting opportunities for a givencoverage class is delegated to the wireless device 200, the rankingapproach simplifies the central scheduling.

The ranking approach can be realized in different ways. According to anembodiment, the backing off is specified according to a physical layerspecification or a medium access layer specification associated with thecommunications network. Below are two examples.

The starting opportunities can be scheduled according to a physicallayer specification by using, e.g., explicit formulas and/or pseudocodethat direct the wireless device 200 to determine the allowed startingopportunities.

The starting opportunities can be defined as part of the wireless devicebehavior by being realized at the medium access layer by specifying thebehaviors in the presence of collision.

FIG. 17 schematically illustrates, in terms of a number of functionalunits, the components of a wireless device 200 according to anembodiment. Processing circuitry 210 is provided using any combinationof one or more of a suitable central processing unit (CPU),multiprocessor, microcontroller, digital signal processor (DSP), etc.,capable of executing software instructions stored in a computer programproduct 410 a (as in FIG. 21), e.g., in the form of a storage medium230. The processing circuitry 210 may further be provided as at leastone application specific integrated circuit (ASIC), or fieldprogrammable gate array (FPGA).

Particularly, the processing circuitry 210 is configured to cause thewireless device 200 to perform a set of operations, or steps, S102-S110,as disclosed above. For example, the storage medium 230 may store theset of operations, and the processing circuitry 210 may be configured toretrieve the set of operations from the storage medium 230 to cause thewireless device 200 to perform the set of operations. The set ofoperations may be provided as a set of executable instructions. Thus theprocessing circuitry 210 is thereby arranged to execute methods asdisclosed herein.

The storage medium 230 may also comprise persistent storage, which, forexample, can be any single one or combination of magnetic memory,optical memory, solid state memory or even remotely mounted memory.

The wireless device 200 may further comprise a communications interface220 for communications at least with the network node 300. As such thecommunications interface 220 may comprise one or more transmitters andreceivers, comprising analogue and digital components and a suitablenumber of antennas for wireless communications and ports for wirelinecommunications.

The processing circuitry 210 controls the general operation of thewireless device 200 e.g. by transmitting data and control signals to thecommunications interface 220 and the storage medium 230, by receivingdata and reports from the communications interface 220, and byretrieving data and instructions from the storage medium 230. Othercomponents, as well as the related functionality, of the wireless device200 are omitted in order not to obscure the concepts presented herein.

FIG. 18 schematically illustrates, in terms of a number of functionalmodules, the components of a wireless device 200 according to anembodiment. The wireless device 200 of FIG. 18 comprises an initiatemodule 210 a configured to perform step S110. The wireless device 200 ofFIG. 18 may further comprise a number of optional functional modules,such as any of an obtain module 210 b configured to perform step S102,an obtain module 210 c configured to perform step S104, a check module210 d configured to perform step S106, and a back-off module 210 econfigured to perform step S108. In general terms, each functionalmodule 210 a-210 e may be implemented in hardware or in software.Preferably, one or more or all functional modules 210 a-210 e may beimplemented by the processing circuitry 210, possibly in cooperationwith the communications interface 220 and/or the storage medium 230. Theprocessing circuitry 210 may thus be arranged to fetch, from the storagemedium 230, instructions as provided by a functional module 210 a-210 e,and to execute these instructions, thereby performing any steps of thewireless device 200 as disclosed herein.

FIG. 19 schematically illustrates, in terms of a number of functionalunits, the components of a network node 300 according to an embodiment.Processing circuitry 310 is provided using any combination of one ormore of a suitable central processing unit (CPU), multiprocessor,microcontroller, digital signal processor (DSP), etc., capable ofexecuting software instructions stored in a computer program product 410b (as in FIG. 21), e.g. in the form of a storage medium 330. Theprocessing circuitry 310 may further be provided as at least oneapplication specific integrated circuit (ASIC), or field programmablegate array (FPGA).

Particularly, the processing circuitry 310 is configured to cause thenetwork node 300 to perform a set of operations, or steps, S202-S204, asdisclosed above. For example, the storage medium 330 may store the setof operations, and the processing circuitry 310 may be configured toretrieve the set of operations from the storage medium 330 to cause thenetwork node 300 to perform the set of operations. The set of operationsmay be provided as a set of executable instructions. Thus the processingcircuitry 310 is thereby arranged to execute methods as disclosedherein.

The storage medium 330 may also comprise persistent storage, which, forexample, can be one of magnetic memory, optical memory, solid statememory or even remotely mounted memory, or a combination of two or moreof these memory types.

The network node 300 may further comprise a communications interface 320for communications at least with a wireless device 200. As such thecommunications interface 320 may comprise one or more transmitters andreceivers, comprising analogue and digital components and a suitablenumber of antennas for wireless communications and ports for wirelinecommunications.

The processing circuitry 310 controls the general operation of thenetwork node 300 e.g. by transmitting data and control signals to thecommunications interface 320 and the storage medium 330, by receivingdata and reports from the communications interface 320, and byretrieving data and instructions from the storage medium 330. Othercomponents, as well as the related functionality, of the network node300 are omitted in order not to obscure the concepts presented herein.

FIG. 20 schematically illustrates, in terms of a number of functionalmodules, the components of a network node 300 according to anembodiment. The network node 300 of FIG. 20 comprises a provide module310 a configured to perform step S202. The network node 300 of FIG. 20may further comprise a number of optional functional modules, such as afurther provide module 310 b configured to perform step S204. In generalterms, each functional module 310 a-310 b may be implemented in hardwareor in software. Preferably, one or more or all functional modules 310a-310 b may be implemented by the processing circuitry 310, possibly incooperation with functional units 320 and/or 330. The processingcircuitry 310 may thus be arranged to fetch, from the storage medium330, instructions as provided by a functional module 310 a-310 b and toexecute these instructions, thereby performing any steps of the networknode 300 as disclosed herein.

The network node 300 may be provided as a standalone device or as a partof at least one further device. For example, the network node 300 may beprovided in a node of the radio access network 110 or in a node of thecore network 120, or even in a node of the service network 130.Alternatively, functionality of the network node 300 may be distributedbetween at least two devices, or nodes. These at least two nodes, ordevices, may either be part of the same network part (such as the radioaccess network 110 or the core network 120) or may be spread between atleast two such network parts. In general terms, instructions that arerequired to be performed in real time may be performed in a device, ornode, operatively closer to the cell than instructions that are notrequired to be performed in real time.

Thus, a first portion of the instructions performed by the network node300 may be executed in a first device, and a second portion of the ofthe instructions performed by the network node 300 may be executed in asecond device; the embodiments disclosed herein are not limited to anyparticular number of devices on which the instructions performed by thenetwork node 300 may be executed. Hence, the methods according to theembodiments disclosed herein are suitable to be performed by a networknode 300 residing in a cloud computational environment. Therefore,although a single processing circuitry 310 is illustrated in FIG. 18 theprocessing circuitry 310 may be distributed among a plurality ofdevices, or nodes. The same applies to the functional modules 310 a-310b of FIG. 20 and the computer program 420 b of FIG. 21 (see below).

FIG. 21 shows one example of a computer program product 410 a, 410 bcomprising computer readable means 430. On this computer readable means430, a computer program 420 a can be stored, which computer program 420a can cause the processing circuitry 210 and thereto operatively coupledentities and devices, such as the communications interface 220 and thestorage medium 230, to execute methods according to embodimentsdescribed herein. The computer program 420 a and/or computer programproduct 410 a may thus provide means for performing any steps of thewireless device 200 as disclosed herein. On this computer readable means430, a computer program 420 b can be stored, which computer program 420b can cause the processing circuitry 310 and thereto operatively coupledentities and devices, such as the communications interface 320 and thestorage medium 330, to execute methods according to embodimentsdescribed herein. The computer program 420 b and/or computer programproduct 410 b may thus provide means for performing any steps of thenetwork node 300 as disclosed herein.

In the example of FIG. 21, the computer program product 410 a, 410 b isillustrated as an optical disc, such as a CD (compact disc) or a DVD(digital versatile disc) or a Blu-ray Disc™. The computer programproduct 410 a, 410 b could also be embodied as a memory, such as arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM), or an electrically erasableprogrammable read-only memory (EEPROM) and more particularly as anon-volatile storage medium of a device in an external memory such as aUSB (Universal Serial Bus) memory or a flash memory, such as a compactflash memory. Thus, while the computer program 420 a, 420 b is hereschematically shown as a track on the depicted optical disk, thecomputer program 420 a, 420 b can be stored in any way which is suitablefor the computer program product 410 a, 410 b.

The inventive concept has mainly been described above with reference toa few embodiments. However, as is readily appreciated by a personskilled in the art, other embodiments than the ones disclosed above areequally possible within the scope of the inventive concept, as definedby the appended list of claims. For example, although at least someembodiments have been described in the context of NB-IoT, theembodiments disclosed herein apply equally to eMTC.

LIST OF ENUMERATED EMBODIMENTS

1. A method for network access of a wireless device to a communicationsnetwork, wherein the wireless device is associated with a coverage classfrom a set of coverage classes, the method being performed by thewireless device, the method comprising:

initiating network access to the communications network by transmittinga preamble sequence for random access on a physical random accesschannel, PRACH,

wherein said network access is initiated during a starting opportunitydefined by the coverage class of the wireless device.

2. The method according to embodiment 1, wherein the startingopportunity is defined by the wireless device backing off frominitiating said network access.

3. The method according to embodiment 2, wherein said backing off isspecified according to a physical layer specification or a medium accesslayer specification associated with the communications network.

4. The method according to embodiment 1, wherein coverage classes in theset of coverage classes are ranked and the wireless device backs offfrom a starting opportunity used by wireless devices in a higher rankedcoverage class.

5. The method according to embodiment 4, further comprising:

checking possible starting opportunities that may be used by the higherranked coverage class; and

backing off from PRACH resources that collide with the higher rankedcoverage class.

6. The method according to embodiment 1, wherein each coverage class inthe set of coverage classes has its own set of possible startingopportunities for network access.

7. The method according to embodiment 1, wherein no two differentcoverage classes in the set of coverage classes share a common startingopportunity.

8. The method according to embodiment 1, wherein each coverage class inthe set of coverage classes is associated with a respective receivedpower level, and wherein those coverage classes whose received powerlevels differ less than a threshold value have at least partlyoverlapping starting opportunities.9. The method according to embodiment 1, wherein all coverage classes inthe set of coverage classes share a common starting subframe forinitiating said network access, and wherein the starting opportunity,during which the network access is initiated, is determined based on thecommon starting subframe.10. The method according to embodiment 1, wherein all coverage classesin the set of coverage classes share a common starting subframe forinitiating said network access and each coverage class in the set ofcoverage classes has a different offset for initiating said networkaccess in relation to said common starting subframe, and wherein saidnetwork access is initiated according to said offset.11. The method according to embodiment 10, wherein each coverage classin the set of coverage classes is associated with a different number ofrepetitions for performing said network access.12. The method according to embodiment 1, wherein each of the coverageclasses in the set of coverage classes is associated with a differentnumber of starting opportunities.13. The method according to embodiment 12, wherein each coverage classin the set of coverage classes is associated with a different number ofrepetitions for initiating said network access, and wherein said numberof starting opportunities for a coverage class with relatively fewerrepetitions is higher than said number of starting opportunities for acoverage class with relatively more repetitions.14. The method according to embodiment 1, wherein said network access isinitiated in a frequency band, and wherein the starting opportunity isdependent on how many of the coverage classes in the set of coverageclasses share the frequency band of the coverage class of the wirelessdevice.15. The method according to embodiment 14, wherein the wireless deviceis provided with information from the network node on how many of thecoverage classes in the set of coverage classes share the frequency bandof the coverage class of the wireless device.16. The method according to embodiment 1, further comprising beingprovided with information from the network node that one coverage classdefines several possible starting opportunities, wherein said severalpossible starting opportunities are distributed in time such that atleast two of said several possible starting opportunities are separatedby a starting opportunity of another coverage class in the set ofcoverage classes.17. A method for enabling network access of a wireless device to acommunications network, wherein the wireless device is associated with acoverage class from a set of coverage classes, the method beingperformed by a network node, the method comprising:

providing a network access configuration to the wireless device, whereinthe network access configuration specifies network access initiation tothe communications network for the wireless device, and wherein thenetwork access configuration specifies a starting opportunity defined bythe coverage class of the wireless device during which network access isto be initiated.

18. The method according to embodiment 17, wherein the startingopportunity is defined by the wireless device backing off frominitiating said network access.

19. The method according to embodiment 18, wherein said backing off isspecified according to a physical layer specification or a medium accesslayer specification associated with the communications network.

20. The method according to embodiment 17, wherein the network accessconfiguration moreover specifies a further starting opportunity, duringwhich wireless devices in a different coverage class in the set ofcoverage classes are to initiate network access.21. The method according to embodiment 17, wherein each coverage classhas its own set of possible starting opportunities for network access.22. The method according to embodiment 17, wherein no two differentcoverage classes in the set of coverage classes share a common startingopportunity.23. The method according to embodiment 17, wherein each coverage classin the set of coverage classes is associated with a respective receivedpower level, and wherein those coverage classes whose received powerlevels differ less than a threshold value have at least partlyoverlapping starting opportunities.24. The method according to embodiment 17, wherein all coverage classesin the set of coverage classes share a common starting subframe forinitiating said network access.25. The method according to embodiment 24, wherein each coverage classin the set of coverage classes has a different offset for initiatingsaid network access in relation to said common starting subframe, andwherein said network access is initiated according to said offset.26. The method according to embodiment 25, wherein each coverage classin the set of coverage classes is associated with a different number ofrepetitions for performing said network access.27. The method according to embodiment 17, wherein each of the coverageclasses in the set of coverage classes is associated with a differentnumber of starting opportunities.28. The method according to embodiment 27, wherein each coverage classin the set of coverage classes is associated with a different number ofrepetitions for initiating said network access, and wherein said numberof starting opportunities for a coverage class with relatively fewerrepetitions is higher than said number of starting opportunities for acoverage class with relatively more repetitions.29. The method according to embodiment 17, wherein said network accessis initiated in a frequency band, and wherein the starting opportunityis dependent on how many of the coverage classes in the set of coverageclasses share the frequency band of the coverage class of the wirelessdevice.30. The method according to embodiment 28, wherein the wireless deviceis provided with information from the network node on how many of thecoverage classes in the set of coverage classes share the frequency bandof the coverage class of the wireless device.31. The method according to embodiment 17, further comprising providingthe wireless device with information that one coverage class definesseveral possible starting opportunities, wherein said several possiblestarting opportunities are distributed in time such that at least two ofsaid several possible starting opportunities are separated by a startingopportunity of another coverage class in the set of coverage classes.32. A wireless device (200) for network access of the wireless device toa communications network, wherein the wireless device is associated witha coverage class from a set of coverage classes, the wireless devicecomprising processing circuitry (210) and a communication interface(220) comprising one or more transmitters, the processing circuitrybeing configured to cause the wireless device to:

initiate network access to the communications network by transmitting,using said communication interface, a preamble sequence for randomaccess on a physical random access channel, PRACH,

wherein said network access is initiated during a starting opportunitydefined by the coverage class of the wireless device.

33. A wireless device (200) for network access of the wireless device toa communications network, wherein the wireless device is associated witha coverage class from a set of coverage classes, the wireless devicecomprising:

processing circuitry (210); and

a computer program product (410 a) storing instructions that, whenexecuted by the processing circuitry, causes the wireless device to:

-   -   initiate network access to the communications network by        transmitting a preamble sequence for random access on a physical        random access channel, PRACH,

wherein said network access is initiated during a starting opportunitydefined by the coverage class of the wireless device.

34. A wireless device (200) for network access of the wireless device toa communications network, wherein the wireless device is associated witha coverage class from a set of coverage classes, the wireless devicecomprising:

an initiate module (210 a) configured to initiate network access to thecommunications network by transmitting a preamble sequence for randomaccess on a physical random access channel, PRACH,

wherein said network access is initiated during a starting opportunitydefined by the coverage class of the wireless device.

35. A network node (300) for enabling network access of a wirelessdevice to a communications network, wherein the wireless device isassociated with a coverage class from a set of coverage classes, thenetwork node comprising processing circuitry and (310) a communicationinterface (320) comprising one or more transmitters, the processingcircuitry being configured to cause the network node to:

provide a network access configuration to the wireless device, usingsaid communication interface, wherein the network access configurationspecifies network access initiation to the communications network forthe wireless device, and wherein the network access configurationspecifies a starting opportunity defined by the coverage class of thewireless device during which network access is to be initiated.

36. A network node (300) for enabling network access of a wirelessdevice to a communications network, wherein the wireless device isassociated with a coverage class from a set of coverage classes, thenetwork node comprising:

processing circuitry (310); and

a computer program product (410 b) storing instructions that, whenexecuted by the processing circuitry, causes the network node to:

-   -   provide a network access configuration to the wireless device,        wherein the network access configuration specifies network        access initiation to the communications network for the wireless        device, and wherein the network access configuration specifies a        starting opportunity defined by the coverage class of the        wireless device during which network access is to be initiated.        37. A network node (300) for enabling network access of a        wireless device to a communications network, the network node        comprising:

a provide module (310 a) configured to provide a network accessconfiguration to the wireless device, wherein the network accessconfiguration specifies network access initiation to the communicationsnetwork for the wireless device, and wherein the network accessconfiguration specifies a starting opportunity defined by the coverageclass of the wireless device during which network access is to beinitiated.

38. A computer program for network access of a wireless device to acommunications network, wherein the wireless device is associated with acoverage class from a set of coverage classes, the computer programcomprising computer code which, when run on processing circuitry of awireless device, causes the wireless device to:

initiate network access to the communications network by transmitting apreamble sequence for random access on a physical random access channel,PRACH,

wherein said network access is initiated during a starting opportunitydefined by the coverage class of the wireless device.

39. A computer program for enabling network access of a wireless deviceto a communications network, wherein the wireless device is associatedwith a coverage class from a set of coverage classes, the computerprogram comprising computer code which, when run on processing circuitryof a network node, causes the network node to:

provide a network access configuration to the wireless device, whereinthe network access configuration specifies network access initiation tothe communications network for the wireless device, and wherein thenetwork access configuration specifies a starting opportunity defined bythe coverage class of the wireless device during which network access isto be initiated.

40. A computer program product comprising a computer program accordingto at least one of embodiments 38 and 39, and a computer-readablestorage medium on which the computer program is stored.

The invention claimed is:
 1. A method performed by a user equipment (UE)to access a communications network, wherein the UE is associated with acoverage class from a set of two or more coverage classes, each coverageclass being associated with transmission of a different number ofrepetitions of a random access preamble sequence for performing networkaccess, the set of two or more coverage classes sharing a frequency bandfor performing network access, the method comprising: generating aSingle-Carrier Frequency-Division Multiple Access (SC-FDMA) randomaccess preamble signal comprising two or more consecutive preamblesymbol groups, each preamble symbol group comprising a cyclic prefixportion and a plurality of symbols occupying a single subcarrier of theSC-FDMA random access preamble signal, wherein the single subcarrier forat least one of the preamble symbol groups corresponds to a firstsubcarrier frequency and the single subcarrier for an immediatelysubsequent one of the preamble symbol groups corresponds to a secondsubcarrier frequency; determining a starting opportunity, at which theuser equipment is to transmit the random access preamble signal, thatavoids collision of the random access preamble signal with any otherrandom access preamble signal transmitted by another user equipmentassociated with a different coverage class, by checking whether one ormore possible starting opportunities related to the coverage class withwhich the user equipment is associated collide with one or more possiblestarting opportunities related to at least one higher coverage class andbacking off from any possible starting opportunities that collideaccording to the checking, wherein the at least one higher coverageclass is associated with a greater number of repetitions of a randomaccess preamble sequence than the number of repetitions associated withthe coverage class of the user equipment; and initiating network accessby transmitting the random access preamble signal at the determinedstarting opportunity and in the shared frequency band.
 2. The methodaccording to claim 1, wherein the starting opportunity for transmittingthe random access preamble signal is based at least on restrictionsgiven by subframes occupied by physical random access channel (PRACH)resources related to the at least one higher coverage class than thecoverage class of the transmitting UE.
 3. The method according to claim1, further comprising receiving one or more parameters indicating atleast one of the following: a periodicity; a subframe containing astarting opportunity; and a number of repetitions per preambletransmission attempt, the method further comprising determining, basedat least in part on the received one or more parameters, the one or morepossible starting opportunities related to the coverage class with whichthe user equipment is associated.
 4. The method according to claim 1,further comprising receiving parameters indicating: a periodicity; asubframe containing a starting opportunity; and a number of repetitionsper preamble transmission attempt, the method further comprisingdetermining, based at least in part on the received parameters, the oneor more possible starting opportunities related to the coverage classwith which the user equipment is associated.
 5. The method according toclaim 3, further comprising receiving a parameter indicating one of aperiodicity; a subframe containing a starting opportunity; and a numberof repetitions per preamble transmission attempt for the at least onehigher coverage class than the coverage class associated with the UE. 6.The method according to claim 3, wherein the received one or moreparameters indicate a subframe containing a starting opportunity, byindicating a starting subframe common to all coverage classes in the setof coverage classes, from which the subframe containing the startingopportunity is determinable.
 7. The method according to claim 6, whereineach coverage class in the set of coverage classes has a differentoffset for initiating said network access in relation to said commonstarting subframe, and wherein said network access is initiatedaccording to said offset.
 8. The method according to claim 3, whereinthe received one or more parameters indicates a number of repetitionsper preamble transmission attempt, and wherein the received one or moreparameters have a unique value for each coverage class in the set ofcoverage classes.
 9. The method according to claim 1, wherein eachcoverage class in the set of coverage classes is associated with UEshaving a respective received power level.
 10. The method according toclaim 1, wherein the two or more coverage classes in the set of coverageclasses are ranked and the UE backs off from initiating network accessat a possible starting opportunity used by UEs associated with a highercoverage class.
 11. The method according to claim 1, wherein saidbacking off comprises determining the starting opportunity to be a nextavailable subframe that contains a physical random access channel(PRACH) and that is not a subframe occupied by PRACH resources relatedto a higher enhanced coverage level.
 12. A base station for enabling auser equipment (UE) to access a communications network, wherein the UEis associated with a coverage class from a set of two or more coverageclasses, each coverage class being associated with transmission of adifferent number of repetitions of a random access preamble sequence forperforming network access, the set of two or more coverage classessharing a frequency band for performing network access, the base stationcomprising processing circuitry and a communication interface comprisingone or more transmitters and one or more receivers, the processingcircuitry being configured to cause the base station to: transmitnetwork access information to the UE, wherein the network accessinformation specifies a starting opportunity at which the user equipmentis permitted to initiate network access by transmitting a random accesspreamble signal in the shared frequency band, subject to a restrictionthat collision of the random access preamble signal with any otherrandom access preamble signal transmitted by another user equipmentassociated with a different coverage class is to be avoided by backingoff from a possible starting opportunity that is related to the coverageclass with which the user equipment is associated but that collides witha possible starting opportunity related to at least one higher coverageclass, wherein the at least one higher coverage class is associated witha greater number of repetitions of a random access preamble sequencethan the number of repetitions associated with the coverage class of theuser equipment; and receive, from the UE and at a starting opportunitythat is or that is backed off from the specified starting opportunity, aSingle-Carrier Frequency-Division Multiple Access (SC-FDMA) randomaccess preamble signal comprising two or more consecutive preamblesymbol groups, each preamble symbol group comprising a cyclic prefixportion and a plurality of symbols occupying a single subcarrier of theSC-FDMA random access preamble signal, wherein the single subcarrier forat least one of the preamble symbol groups corresponds to a firstsubcarrier frequency and the single subcarrier for an immediatelysubsequent one of the preamble symbol groups corresponds to a secondsubcarrier frequency.
 13. A user equipment (UE) configured to access acommunications network, wherein the UE is further configured to beassociated with a coverage class from a set of two or more coverageclasses, each coverage class being associated with transmission of adifferent number of repetitions of a random access preamble sequence forperforming network access, the set of two or more coverage classessharing a frequency band for performing network access, the userequipment comprising processing circuitry and a communication interfacecomprising one or more transmitters, the processing circuitry beingconfigured to cause the UE to: generate a Single-CarrierFrequency-Division Multiple Access (SC-FDMA) random access preamblesignal comprising two or more consecutive preamble symbol groups, eachpreamble symbol group comprising a cyclic prefix portion and a pluralityof symbols occupying a single subcarrier of the SC-FDMA random accesspreamble signal, wherein the single subcarrier for at least one of thepreamble symbol groups corresponds to a first subcarrier frequency andthe single subcarrier for an immediately subsequent one of the preamblesymbol groups corresponds to a second subcarrier frequency; determine astarting opportunity, at which the user equipment is to transmit therandom access preamble signal, that avoids collision of the randomaccess preamble signal with any other random access preamble signaltransmitted by another user equipment associated with a differentcoverage class, by checking whether one or more possible startingopportunities related to the coverage class with which the userequipment is associated collide with one or more possible startingopportunities related to at least one higher coverage class and backingoff from any possible starting opportunities that collide according tothe checking, wherein the at least one higher coverage class isassociated with a greater number of repetitions of a random accesspreamble sequence than the number of repetitions associated with thecoverage class of the user equipment; and initiate network access bytransmitting the random access preamble signal at the determinedstarting opportunity and in the shared frequency band.
 14. The UEaccording to claim 13, wherein the processing circuitry is configured todetermine the starting opportunity for transmitting the random accesspreamble signal at least based on restrictions given by subframesoccupied by physical random access channel (PRACH) resources related tothe at least one higher coverage class than the coverage class of thetransmitting UE.
 15. The UE according to claim 13, wherein thecommunication interface further comprises one or more receivers and theprocessing circuitry is further configured to cause the UE to: receive,using the communication interface, one or more parameters indicating atleast one of the following: a periodicity; a subframe containing astarting opportunity; and a number of repetitions per preambletransmission attempt, and determine, based at least in part on thereceived one or more parameters, the one or more possible startingopportunities related to the coverage class with which the userequipment is associated.
 16. The UE according to claim 13, wherein thecommunication interface further comprises one or more receivers and theprocessing circuitry is further configured to cause the UE to: receive,using the communication interface, parameters indicating: a periodicity;a subframe containing a starting opportunity; and a number ofrepetitions per preamble transmission attempt, and determine, based atleast in part on the received parameters, the one or more possiblestarting opportunities related to the coverage class with which the userequipment is associated.
 17. The UE according to claim 15, wherein thecommunication interface further comprises one or more receivers and theprocessing circuitry is further configured to cause the UE to receive,using the communication interface, a parameter indicating one of aperiodicity; a subframe containing a starting opportunity; and a numberof repetitions per preamble transmission attempt for the at least onehigher coverage class than the coverage class associated with the UE.18. The UE according to claim 15, wherein the received one or moreparameters indicate a subframe containing a starting opportunity, byindicating a starting subframe common to all coverage classes in the setof coverage classes, from which the subframe containing the startingopportunity is determinable.
 19. The UE according to claim 18, whereineach coverage class in the set of coverage classes has a differentoffset for initiating said network access in relation to said commonstarting subframe, and wherein said network access is initiatedaccording to said offset.
 20. The UE according to claim 15, wherein thereceived one or more parameters indicates a number of repetitions perpreamble transmission attempt, and wherein the received one or moreparameters have a unique value for each coverage class in the set ofcoverage classes.
 21. The UE according to claim 13, wherein eachcoverage class in the set of coverage classes is associated with UEshaving a respective received power level.
 22. The UE according to claim13, wherein the processing circuitry is further configured to cause theUE to generate a preamble signal in which said plurality of symbols ofeach preamble symbol group are identical.
 23. The UE according to claim13, wherein the processing circuitry is further configured to cause theUE to generate a preamble signal for which the first and secondsubcarrier frequencies are related by a 1-subcarrier frequency shift ora 6-subcarrier frequency shift.
 24. The UE according to claim 13,wherein UE is configured to be associated with a coverage classassociated with two or more repetitions, and the processing circuitry isfurther configured to cause the UE to generate a preamble signal forwhich the first and second subcarrier frequencies are related by apseudo-random frequency shift.
 25. The UE according to claim 13, whereinthe shared frequency band is a 12-subcarrier frequency band.
 26. The UEaccording to claim 13, wherein the two or more coverage classes in theset of coverage classes are ranked and the processing circuitry isconfigured to cause the UE to back off from initiating network access ata possible starting opportunity used by UEs associated with a highercoverage class.
 27. The UE according to claim 26, wherein saidbacking-off is implemented on a medium access layer level.
 28. The UEaccording to claim 13, wherein said backing off is implemented on amedium access layer level.
 29. The UE according to claim 13, whereinsaid backing off comprises determining the starting opportunity to be anext available subframe that contains a physical random access channel(PRACH) and that is not a subframe occupied by PRACH resources relatedto a higher enhanced coverage level.